In tape recording and reproducing machines, tape is transported back and forth between a supply reel and a takeup reel. To this end, a tape machine thus typically includes a tape reel drive system and a drive capstan, wherein each component is driven by separate electric motors. The supply and takeup reel motors generally are controlled by respective reel servos to maintain a predetermined tension on the tape, while the capstan is controlled by a capstan servo to selectively move the tape. The tape is moved at a predetermined slower speed during the normal recording and reproducing modes, and is moved at very rapid speed of the order of 300 inches per second during a shuttle mode of operation.
The electrical system of the tape transport apparatus includes a power supply that provides the necessary voltages for powering the reel drive system. In the event of a power failure, the reel drive motors decelerate to a stop as a result of frictional forces. These frictional forces tend to subject the tape to substantially varying tensile loads, which loads are undesirable because they can physically damage the tape or cause pressure erasure of magnetic signals recorded on the tape.
Additionally, the inertia of the takeup reel may be less than that of the supply reel because of differences in the quantity of tape stored on the reels at the time of a power failure, or because of differences in reel size. Thus, during a power failure, the supply reel may decelerate more slowly than that of the takeup reel, whereby the tape is unwound from the supply reel at a rate greater than the rate that the takeup reel is receiving tape. As a result, a slack tape loop may form and tape may be spilled instead of being taken up on the takeup reel during the stopping process. The possibilities of tape spillage from the supply reel and resulting tape damage become especially likely during shuttle operations because of the very high tape speeds involved.
Typical of tape reel motor systems in existing less sophisticated tape recording and reproducing machines are those equipped with mechanical brakes, which brakes are enabled by a solenoid in the event of a power failure. Typically, the mechanical brakes operate differentially in that they exert greater braking torque upon the motor of the reel which supplies tape than upon the motor of the reel which receives tape. In other recording and reproducing machines, dynamic brakes may be employed in the tape reel motor systems, wherein the motor current resulting from the back electromotive force (emf) of the motor produces dynamic braking of the motor upon the occurrence of a power failure.
While such brakes have generally been satisfactory for use in longitudinal audio and quadruplex video recording and reproducing machines, they provide less than satisfactory performance in present more sophisticated helical wrap recording and reproducing machines. The helical wrap machines generally have high tape wrap angles around fixed guides, which configuration causes a prohibitive build up of tension in the tape path extending from the supply reel to the takeup reel. This excessive tension build up, when combined with situations wherein reels having large differences in inertia are used, and wherein varying tape pack diameters occur, causes corresponding continuously varying differences in the dynamic braking requirements. To overcome these problems, a sophisticated dynamic braking system has been developed recently wherein means are provided for controlling the application of braking forces during the entire deceleration of a reel motor until it comes to a stop. Such a dynamic braking system for a tape transport apparatus is disclosed, for example, in U.S. Pat. No. 4,481,449 to D. R. Rodal, issued Nov. 6, 1984, and assigned to the same assignee as this application.
The circuit of the above patent includes a motor drive amplifier (MDA) coupled to drive a DC motor bidirectionally, to thus selectively and controllably rotate the supply and takeup reels in the forward and reverse directions. The MDA is an H-type switching circuit, commonly known as a full wave bridge circuit, that includes a multiplicity of transistor switches and associated diodes. During normal operation of the tape drive with full power available, the transistor switches of the MDA are pulsed to effect a controlled rotational speed of the tape reels. The duty cycles of the pulses of current are varied in accordance with error signals provided by sensing tape tension changes and the motor torque or current. The power fail circuit includes, among other things, a power supply interrupt means that is responsive to a power fail signal, a missing cycle alarm generator and a bidirectional motor drive amplifier including the bridge switching circuit arrangement of previous mention. The system allows controlled application of dynamic braking to the reel drive motors by selectively interrupting the braking process at controlled intervals to maintain a predetermined tape tension during the deceleration of tape movement to a stop. However, the power fail servo system of the patent is relatively complex and is applicable to the control of tape reels and video cassettes of varying sizes and weights that are used in relatively elaborate, computer controlled, tape transport apparatus.
The invention overcomes the shortcomings of the mechanical braking systems, and the complexity of the sophisticated dynamic braking system of previous discussion, while providing a simple and inexpensive circuit for effecting controlled dynamic braking of a tape reel drive motor in the event that a sudden loss of power is experienced by an associated tape transport. The relatively simple dynamic braking circuit particularly is applicable to tape transports employing relatively small and light tape reels, and to tape transports capable of handling tape cassettes of different sizes.
To this end, the present dynamic braking system comprises a control circuit that is activated when a tape transport apparatus experiences power failure. The control circuit is coupled between the power supply and two motor drive amplifiers associated respectively with the tape supply and takeup reel drive motors. The control circuit includes a storage capacitor which is charged during normal operation with power on, and an operational amplifier and a field effect transistor (FET) coupled to the capacitor. In the event of power failure, the failure is detected, and the operational amplifier is enabled, which causes the FET to conduct. The capacitor supplies power to the operational amplifier and the FET, to maintain the FET in a constant current conducting mode until the motor stops. The kinetic energy of the rotating motor, which decelerates upon loss of power, generates a back electromotive force (emf) and thus a dynamic braking current that is dissipated as heat via the FET to supply the energy necessary to brake the motor to a stop. A voltage is fed back to the operational amplifier which adjusts the gate voltage of the FET to maintain its constant current conducting mode, and thus to maintain the dynamic braking current relatively constant. As a result, controlled deceleration and dynamic braking of the drive motor for the tape supplying reel is accomplished with a relatively simple circuit.